1. Technical Field
The technical field of the present invention is the thermal protection of post-combustion jet nozzles used in aeronautic gas turbine engines such as turbojets.
2. State of the Prior Art
Post combustion jet nozzles for gas turbine engines have already been the subject of various constructions according to the prior art; Conventional jet nozzles placed at the exit of the post-combustion chamber are known. These are normally fitted with a protective thermal shroud circumscribing a gas duct that allows a flow of hot gases where the burnt gases are situated, and an air duct allowing a secondary flow of air containing relatively fresh air. Naturally, said protective thermal shrouds must be adapted to withstand considerable thermal constraints, because said shrouds are in the proximity of the burnt gases that are at a very high temperature.
As can be seen in FIG. 1, which shows a partial cross-section of a jet nozzle according to the prior art, said jet nozzle has a primary gas duct 20 surrounded by a secondary air duct 21. Said two ducts are circumscribed by a multi-perforated protective thermal shroud 22, suited to allowing the passage of air which acts to cool it, from the secondary air duct 21 towards the primary gas duct 20.
Said jet nozzle also comprises a leak tight part 23 that prevents the passage of the fresh air contained in the secondary air flow 21 in the direction of the jet nozzle dampers 24. Said jet nozzle dampers 24 are directly connected to control elements 25, at least part of which are located around the secondary air duct 21, at the level of a downstream end 22a of the protective thermal shroud 22.
However, it has been observed that when jet nozzles of the prior art are used, during full thrust operation (military power), in other words when the dampers 24 of the jet nozzle are in the closed position, the secondary air flow is subjected to an important rise in temperature at the level of the downstream end 22a of the protective thermal shroud 22.
In fact, the hot gases found in the gas flow 20 penetrate the secondary air flow 21, without the relatively fresh gases found in this flow managing to conserve a sufficiently low temperature so as not to create local overheating that could damage the control elements 25 for the dampers 24. Still referring to FIG. 1, the arrows symbolise the movements of the hot gases retained in the downstream direction and circulating in the direction of the secondary air flow 21 through the multi-perforated protective shroud 22. The consequences of this localised overheating in the secondary air flow 21 are that the parts of the jet nozzle, particularly the titanium casing, as well as the damper control elements located at the downstream end 22a of the protective thermal shroud 22, have an operational lifetime that is altered due to the rise in temperature.
Said overheating, directly linked to a high deflection angle of the dampers 24, creates resulting deformations on the parts located around the secondary air flow 21 as well as on those parts making up the control elements 25 for the dampers 24.
The object of the present invention is therefore to overcome, at least partially, the various disadvantages cited above by proposing a cooling system for a gas turbine engine post-combustion jet nozzle, limiting the effects of overheating of the secondary air flow, particularly when the gas turbine engine is operating at full thrust.
To achieve this, the invention is related to a cooling system for a post-combustion jet nozzle, said jet nozzle comprising a primary gas duct allowing a primary flow of gas, a secondary air duct allowing a secondary flow of air, said secondary air duct surrounding the primary gas duct and separated from it by a protective thermal shroud, said secondary air duct having a downstream end, dampers surrounding an output section of the primary gas duct, the cooling system comprising a protective thermal shell in the secondary air duct, at the downstream end of said duct. According to the invention, the protective thermal shell bears an annular diaphragm extending out in front of the dampers and being provided with support sectors and inter-sector zones equipped with slots, said inter-sector zones defining spaces between the diaphragm and the protective thermal shroud.
The main advantage of this invention is that it overcomes the overheating problems of the secondary air flow at the level of the downstream end of the secondary air duct, thus avoiding deformations to the parts located in this region. Apart from this absence of deformation, the cooling system according to the invention, by using a supplementary thermal protection in the secondary air duct, also participates in slowing down the reduction in the operational lifetime of the elements located in this critical region.
Moreover, the invention advantageously comprises means that make it possible to ventilate the jet nozzle dampers by means of the secondary air flow in the direction of said dampers. Previously, the air duct was sealed by means of a leak tight part that completely prevented the ventilation of said dampers. Its replacement by this diaphragm thus makes it possible for the air coming from the secondary air flow to pass in the direction of the jet nozzle dampers, and it does this whatever the temperature of the different parts making up the jet nozzle. The ventilation of the dampers favoured in this way thus makes it possible to avoid damaging said dampers due to temperatures that are too high to withstand, but also to increase the operational life of all of said parts.
In a preferential manner, the diaphragm is fastened onto the protective thermal shell, and allows a displacement of the protective thermal shroud in relation to the support sectors.
Said formed support sectors support the shroud and allow said protective thermal shroud to be accurately centred.
According to a preferred embodiment of the invention, the protective thermal shell is provided with fastening supports bearing on a casing circumscribing the outside of the secondary air duct.
Other characteristics and advantages of the invention will become clear from the detailed, non-limitative description that follows.